1. Field of the Invention
The present invention relates to means for monitoring communications networks, in particular arrangements for detecting faults affecting traffic signal paths.
2. Related Art
It is desirable in most, if not all, communication networks, that there should be some form of in-service monitoring, so that failure of a part of the network is detected promptly and can be dealt with. There are many ways in which a failure may be detected and some form of indicator is normally required.
Usually, if a piece of equipment fails, it emits an alarm. The alarm may indicate what kind of failure has occurred.
Downstream of the failed equipment, in a signal path, further pieces of equipment may detect that a failure has occurred upstream, for instance because of signal loss. These downstream pieces of equipment might also then each issue an alarm. To prevent misleading generation of multiple alarms from downstream equipment suffering signal loss or perturbation, it is known for the first piece of equipment downstream from a fault to emit instead an Alarm Indication Signal (AIS). This is not itself an alarm but indicates that an alarm has been generated. It then prevents all the following equipment in the signal path from generating their own alarms. Without the AIS, a fault might otherwise be difficult to locate and diagnose.
Digital communications systems used by network operators have an established practice in which a loss of signal detected by a piece of equipment downstream from a fault in a path causes the equipment to send an AIS. This is normally in the form of a continuous stream of digital "1"s. The AIS could be in any other form defined and recognised by the network in question, but digital "1"s have been found particularly appropriate.
The AIS must be detectable and generally AIS detection criteria are set at a threshold of a given number of continuous digital "1"s, or a given density of "1"s over a certain span of bits. The AIS may be set at a relatively high level so that detection is not assumed after only a short string of "1"s. Because network performance is important to both customer and network operator, both the time and the number of incidents for which an AIS is triggered may be recorded.
Communication paths are available at various bit rates or rates of data transmission to suit the customer's requirements. At the high data rates there is sometimes a deterministic frame structure. A deterministic frame structure is one in which a source sends a signal with a well defined structure at the binary level in the form of overhead plus payload. At lower data rates, such as at 64 Kbit/sec, there tends to be no generic deterministic frame structure.
`Structured` services usually take the form of a standardised path overhead (such as a periodic framing pattern perhaps with a CRC-type function which can be used for monitoring purposes) and a well defined payload area for the, usually non-deterministic, customer traffic signal. An example of a structured signal is the G.704/G.706 ITU recommended frame structure for the 2048 kbit/s network layer path signal. In this example the path overhead is defined in Timeslot 0 (which contains, amongst other functions, both a frame alignment signal and a CRC), and the payload the customer can use is, in most cases, the remaining 31 Timeslots of the frame in some format (thus giving the customer a maximum aggregate 1984 kbit/s payload channel).
Many network operators offer services which take the form of giving the customer the whole bit rate of the digital path in question. For example, this could be a n.times.64 kbit/s path (where n=1,2,3 . . . to some limit perhaps) or a higher bit rate hierarchical network layer, such a 2048 kbit/s or 34368 kbit/s path. This creates a problem for the network operator offering such a service because there is no tgeneric frame structure requirement imposed on the signal at the binary level (either by the customer or the network operator) which can be used by the network operator to monitor the end-to-end performance of the path at the binary level. In this specification such service offerings will be referred to as `unstructured`.
A particular problem with unstructured services is thus that the network operator might not be quickly (if at all) aware of path failures. In some cases this information is first provided by the customer.
In public telecommunications networks systems where quality of service and continuity of service are specified to very high levels, if not guaranteed, an extra path can be provided for back-up purposes. This extra path will generally take a different route from the primary path to assist in providing the level of service being supplied to the customer. In a standard service, however, only one traffic carrying path will be provided on an end to end basis because the provision of an additional end to end path would significantly increase cost .
For reasons of economy, it is therefore desirable that capacity for monitoring a standard service be derived purely from the path which carries the traffic. For instance, as mentioned above, it is known to use an Alarm Indication Signal (AIS), which is commonly a signal comprising all "1"s. This particular all "1"s AIS is widely used (and defined in the international standards bodies such as the ITU) because for one reason it is easily implemented in practice in a wide range of terminal equipment.
However, there is a problem in that an AIS cannot be used in the traffic path without restricting the form of traffic carried in some way so that it does not inadvertently mimc an AIS.